Power transistors are used as switches in power electronic systems. The switches alternate between conducting a high current in the on-state, and blocking a high voltage in the off-state. Two of the most important figures of merit for power switches are low power losses during forward conduction and low power losses during switching between on and off. Low power losses are beneficial because they enable energy savings, and because more compact systems can be constructed as the heat dissipation cause by the power losses is reduced.
Silicon carbide (SiC) bipolar junction transistors (BJTs) have low power losses during conduction and switching and are therefore useful as switches in power electronic systems. Transitional silicon (Si) power transistors such as MOSFETs and Insulated Gate Bipolar Transistors (IGBTs) cannot match the power losses of SiC BJTs for voltage ratings from about 1200 V and above. There are also other SiC power transistors such as MOSFETs and Junction Field Effect Transistors (JFETs) and both of these transistor types have relatively low power losses. The MOSFET has, however, limited oxide reliability and high channel resistance which causes additional power losses during forward conduction. The JFET is a so-called normally-on device, and this is a disadvantage in many power electronic systems, because it is a safety concern if the JFET drive circuit fails.
In Proceedings of the 19th International Symposium on Power Semiconductor Devices and Ics, p. 293-6, 2007, by S. Balachandran et al. it is described that SiC BJTs of the NPN type have been successfully developed and low on-state voltages have been shown for BJTs capable of maximum voltages up to 6 kV.
In IEEE Electron Device Letters, Vol. 26, No. 3, 2005, by S. Krishnaswami et al., it is shown that large areas of SiC BJTs for 30 A and 1000 V have been developed with a current gain of about 40 and a low forward voltage drop of only 0.6 V at a current density of 100 A/cm2.
The best SiC BJTs are fabricated with an epitaxial NPN structure and the base-emitter and base-collector junctions are terminated by dry etching of SiC to form so-called mesa structures. A SiC BJT, with high current capability of several amps, contains many inter-digitated emitter fingers spread over a large area of several mm2. Key factors for obtaining state-of-the-art SiC BJTs with optimum breakdown voltage and low power losses are; an efficient high voltage junction termination, bulk SiC material with low defect concentrations, low-resistive ohmic contacts to both n-type and p-type SiC, and an efficient surface passivation.